18k hgf would stand for 18k heavy gold filled ( not solid gold )
HgF
Mercuryfluoride
Molecular formula is Hg2F2 . The empirical formula is HgF . For empirical formulas you reduce the numbers as far as possible.
18KT is gold that has been alloyed with a high percentage of copper and silver. 18Kt gold with more copper blended with it looks more red, and 18KT blended with silver alone looks more green.
18KT gold electroplated means that this item is gold plated. Your item is most likely 18KT gold over silver.
It could mean 18 karat "hollow gold filled". I'm not familiar with a manufacturer's hallmark that is "HGF".
Heavy gold filled
It means 18kt gold electroplated.
It is worth nothing at all. HGE refers to heavy gold electroplated. Though 18KT refers to 18 karat gold or 75 percentage gold, 18KT HGE is worth nothing.
it is 18kt gold
An angle whose measure is 20 degrees!